The present invention relates generally to push-pull cable assemblies used to transmit mechanical motion between remote control and controlled stations. More particularly, the present invention relates to an improved concept by which the core of a push-pull control cable assembly interacts with a novel force conversion unit at the control and controlled stations to comprise an improved concept for the transmission of mechanical motion therebetween. Specifically, the present invention relates to an improved concept by which to convert selective rotary motion to linear reciprocation of the core in a push-pull control cable assembly for transmitting mechanical motion to one or more remote locations where the reciprocating motion is reconverted to rotary motion.
The most basic form of motion transfer between locations remote from one another has for years conventionally utilized rope (or cable) and pulleys, or other "balanced" remote control systems. Balanced remote control systems are almost as old as recorded history, and, when used in conjunction with steering mechanisms such as found on old sailing ships, have traditionally comprised a wheel and shaft with a rope drum to provide the required movement of the rope necessary to operate the rudder or other controllably driven components. The appellation "balanced" appears quite appropriate when it is realized that the mechanical motion transmitting ropes, or cables, form a closed system because of their inability to relay mechanical motion by other than tensile stresses.
Balanced systems are still widely used in a host of environments quite diverse from marine steering systems. However, balanced systems are quite bulky and cumbersome. Moreover, misalignment between the guiding pulleys, rope drums or any of the other components can cause excessive binding and wear to the system.
The advent of push-pull control cable assemblies provided, in a single cable, the necessary structure for effecting remote control by the application of either tensile or compressive forces. The push-pull control cable assembly thus provides a transfer device which overcomes the difficulties incident to balanced systems and is particularly easy to install without requiring specialized engineering or mechanical ability.
Push-pull control cable assemblies, generally, are well known to the art as devices capable of transmitting mechanical motion in either direction when at least the ends of the cable casing are satisfactorily clamped in position. Although the prior art knows many constructions for push-pull control cable assemblies, an exemplary form employs a casing constructed with a plurality of wires laid contiguously in a long pitched helix around the outer periphery of a flexible, plastic tube. The helically arranged wires of the casing are maintained in their proper position solely by a plastic cover in smaller cables and by a reinforcing spread helix of wire, or flat metallic ribbon, in conjunction with a plastic cover, in larger cables. This construction provides a casing with the required flexibility and permits reciprocation of the cable core therein with the maximum efficiency.
The plastic tube which comprises the innermost element of the cable casing not only acts as a bearing for the core of the cable assembly which is slidable within the casing, but also acts to protect the casing wires from any natural elements gaining access to the passageway through the interior of the cable casing.
The plastic cover which comprises the outermost element of the cable casing not only acts as a structural member to retain the casing wires in their helically coiled configuration but also acts as a protective member to shelter the wires from the natural elements having access to the exterior of the casing.
Actuation of the core element in a push-pull control cable assembly, however, requires something more than the prior rope drum of the balanced system, and complex actuating heads have been developed to translate rotative motion into the linear motion of the push-pull control cable core element.
Fittings are provided at each end of the cable casing to provide a means for securing the control cable in operative position and to seal, as well as possible, the ends of the wires from the elements.
At least two major types of approach have been employed to effect a mechanical connection between the core element of the push-pull control cable assembly and the control mechanism by which linear motion is converted into rotary motion, and vice versa.
One approach has been to connect the core element to a push rod that is telescopically slidable within a sleeve that is connected to the casing of the push-pull control cable assembly. In this configuration the sleeve offers some support to the core element as it extends beyond the casing, and the push rod possesses the necessary columnar rigidity to transmit both tensile and compressive loads exteriorily of both the cable casing and the terminal sleeve. This arrangement is widely employed, works quite well and, because it possesses only two disadvantages, is largely accepted. The two major drawbacks are the space required operatively to mount a push rod and sleeve -- both of which must be sufficiently longer than the required throw to be effected by the push rod in order for the push rod to remain directionally supported by the sleeve even when the push rod has been extended outwardly of the sleeve to its maximum extent -- and the inability of the sleeve completely to support the core element against "snaking." In this latter regard it must be appreciated that in order effectively to secure the core element to the push rod at least a portion of the push rod slidably received within the sleeve is normally of a diameter greater than the diameter of the core element. As such, the core element is not fully supported within the sleeve and the core element will snake under compressive loading. This snaking reduces the tactile sensitivity of the control mechanism and induces backlash.
A second approach has been to provide a control mechanism within which the core element would be more fully supported. This approach is exemplified by a construction in which the core element is attached to the outer periphery of a drum-like, internally toothed gear plate. The drawback to this arrangement lies in the fact that, as the gear plate is rotated to apply compressive stresses to the core element, it tends to move radially away from the gear plate. Various backings have been tried to restrain the core element against this radially outward movement, but all have some deficiencies which make them unsatisfactory for heavy loading and maximum rotation. One backing comprises a fixed plastic ring encircling the periphery of the gear plate which confines the core element against moving radially outwardly, but this arrangement has been found to impart excessively high frictional resistance against movement of the core element by the application of compressive forces.
Another backing devised in an attempt to obviate the difficulties attendant upon applying compressive forces by the gear plate type of rotary actuator utilizes a plurality of rotatably mounted rollers circumferentially spaced about the gear plate and engagable by the core element as it moves radially under the application of compressive forces induced by the rotation of the gear plate. With this type actuator the core element tends to flex severely between its points of contact with the rollers causing backlash, and this, together with the friction attendant upon the spindle mounting of the rollers, deprives such constructions of the desired efficiency.